() => {
    //函数类型声明方式1: 单独制定函数和返回值的类型
    function fn1(a, b) {
        return "1";
    }
    const fn2 = (a, b) => {
        return 1;
    };
    const fn3 = (a, b) => {
        return "1";
    };
    const fn4 = (a, b) => {
        return 1;
    };
};
